Spectrum availability is limited and the usage of this resource is very high. Many different technologies co-exist in one region and in addition, different regions allocate the spectrum in a different manner.
Wireless communications is one of the technologies with high spectrum demand. New spectrum is constantly being allocated for this technology, either “free” or previously allocated for another use spectrum. The more bands are specified, the less frequency separation (guard bands) is possible to allocate between the systems (e.g. different cellular operator networks) in order to guarantee protection from interference between the systems. This small separation may lead into interference issues in case the uplink (UL) and downlink (DL) direction are not sufficiently far apart in frequency. There is a need to allow for deployment of UL and DL nearby in frequency while still keeping good performance from a system point of view.
Operating Bands
The standardization group of The Third Generation Partnership Project (3GPP) specifies requirements for 3GPP technologies. In particular, it standardizes the Universal Mobile Telecommunications System, UMTS, Terrestrial Radio Access (UTRA) and evolved UTRA (E-UTRA) technologies.
Technical Specifications (TS) are being developed by 3GPP, the radio requirements for both User Equipment (UE) and Base Station (BS) among them. In these specifications, the concept Operating Band is defined. An operating band is a set of contiguous frequencies allocated for radio device, e.g. UE, and/or base station transmission.
An operating band can be specified as a paired band or an unpaired band. A part of the Operating band is defined for UL (UE transmission/BS reception) and another part is designated for DL (UE reception/BS transmission) for paired or FDD (Frequency Division Duplex) bands. Both UL and DL function at the same time in case of FDD. On the other hand, both UL and DL share the same frequencies in case of unpaired or TDD (Time Division Duplex) bands. As an example Band 1 or Band I is an FDD band specified by 3GPP for both UTRA and E-UTRA technologies, UL E-UTRA Band 1 or UTRA FDD Band I is specified as 1920-1980 MHz and DL E-UTRA Band 1 or UTRA FDD Band I is defined as 2110-2170 MHz. An example of a TDD band is E-UTRA Band 38 or UTRA TDD Band d), which is defined as 2570-2620 MHz.
Unwanted Emissions
3GPP specifies requirements to ensure co-existence among different technologies. In particular, co-existence requirements to ensure deployment of E-UTRA and UTRA technologies in the same geographical area are included as part of the radio requirements. This is to ensure BS-BS, UE-UE UE-BS and BS-UE co-existence.
TS 25.101, 25.102 and 36.101 contain the Radio Core requirements for the UE. These specifications include, among others, UE emission requirements to ensure co-existence between devices implementing 3GPP technologies. 3GPP has specified a “standard UE spurious emissions for co-existence” equal to −50 dBm/MHz for E-UTRA co-existence, or corresponding −60 dBm/30.84 MHz for UTRA for DL protection. This corresponds to an UL requirement for the device to protect DL frequencies allocated for E-UTRA or UTRA and the level has been derived to ensure co-existence between radio devices in close proximity in terms of victim performance.
For an aggressor UE, it is feasible to fulfil −50 dBm/MHz of the standard UE spurious emissions for co-existence at a large frequency separation from its UL operating band. However, the closer this is required from the edge of the deployed carrier (i.e. the closer the frequency channels of the aggressor UE and a victim UE is), the more challenging it becomes to fulfil the −50 dBm/MHz requirement.
When there is a large frequency separation between the aggressor UL operating band and the victim DL operating band for which −50 dBm/MHz is required, the PA emissions can be further attenuated by the duplexer or RF filter. A-MPR (Additional Maximum Power Reduction) or power back-off may also be allowed for specific UL Operating Bands for which the filter attenuation is not enough to fulfil the co-existence requirements.
With the increase of spectrum being allocated to 3GPP technologies, there is a trend to allocate operating bands close to each other in frequency. As an example, E-UTRA Band 27 and E-UTRA Band 28 were introduced in the release 11 (Rel-11) of the 3GPP specifications. E-UTRA Band 27 UL/DL is defined as 807-824/852-869 MHz and E-UTRA Band 28 UL/DL is specified as 703-748/758-803 MHz. E-UTRA Band 27 UL is allocated at 4 MHz from E-UTRA Band 28 DL. In order to protect E-UTRA Band 28 DL, E-UTRA Band 27 devices are allowed to apply certain back-off. However, the back-off becomes quite large to fulfil the standard −50 dBm/MHz and the spurious emissions for E-UTRA Band 28 DL protection were relaxed up to −32 dBm/MHz for the closest frequencies to E-UTRA Band 27 UL. A problem is that with these relaxed out-of-band emissions (OOBE) levels in some bands, the co-existence of systems in nearby frequencies is not possible when UE-to-UE interference occurs. This means that the OOBE requirements should be made much more stringent to protect UEs from this type of interference or to devise new mechanisms to overcome the UE-to-UE type interference. Since toughening the OOBE requirements on UEs will incur high costs (putting more filters etc.), this is not a feasible solution.
UE Measurements
In order to assess the channel conditions of an active link, several measurements are done at both the UE and network level in order to maintain a radio connection of adequate quality. To inform the BS about the channel quality, all UEs periodically perform measurements on the Reference Signals (RS) transmitted by the BSs and send reports to the BS. Such measurements include: the Reference Signal Received Power (RSRP), Channel Quality indicator (CQI), Reference Signal Received Quality (RSRQ), and Received Interference Power. These measurement reports are then used by the BS to validate the channel conditions, measure interference levels and instigate resource optimization and handover procedures.
These measurements are described in 3GPP TS 36.214.